Trends and variability of heat waves in Europe and the association with large-scale circulation patterns

Abstract

Heat waves, defined by consecutive days of abnormally high temperatures exceeding local or regional norms, have been extensively studied during the summer season. However, their characteristics and driving forces in mid-to-late spring (April and May) and early autumn (September) remain poorly understood. This study employed Empirical Orthogonal Functions and composite analysis to investigate the frequency, trend, and spatio-temporal variability of heat waves across Europe from 1921 to 2021. Our analysis reveals a significant upward trend in heat wave occurrences across most European regions, with a notable surge in the last three decades, beginning in the early 1990s. Furthermore, an increase in heat wave events has been observed in both mid-to-late spring and early autumn. The decade of 2011-2021 exhibited the highest number of recorded heat waves, with particularly intense periods, in terms of both frequency and spatial extent, occurring in 2003, 2007, 2012, 2015, and 2018. The most pronounced rise in heat wave frequency is evident in southern regions, including Spain, France, and Italy, extending through Central Europe and the Fennoscandian Peninsula. Southern and eastern regions display the most significant increase compared to previous periods. We identified three distinct blocking patterns potentially influencing the observed spatial and temporal variability of heat waves across spring, summer, and autumn. The first pattern corresponds to the positive phase of the North Atlantic Oscillation. The second is characterized by a blocking pattern over Fennoscandia (Scandinavian blocking). The third exhibits a high-pressure system in the west and low-pressure anomalies in the east. These findings contribute to a more comprehensive understanding of the seasonal characteristics, underlying mechanisms, and driving forces of heat waves in Europe.